This invention relates to an image transfer belt for use in a variable image size offset printing system, and more particularly to a continuous belt construction for transferring inked images from a variable size printing plate to a substrate.
In traditional offset printing processes, the printing press includes a plate cylinder or drum, a blanket cylinder, and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate wrapped around the cylinder, which printing plate defines an image to be printed. The blanket cylinder carries a printing blanket having a flexible surface which contacts the printing plate in a nip between the plate cylinder and blanket cylinder. A web or substrate to be printed is passed through a nip between the blanket cylinder and impression cylinder. Ink is applied to the surface of the printing plate which transfers the inked image to the surface of the blanket at the nip between the plate and blanket cylinders. The inked image is then transferred (offset) to the web at the nip between blanket cylinder and impression cylinder.
Typically, the printing plate cylinder is of a fixed diameter, and the printing blanket cylinder is selected to match this diameter. This diameter fixes the maximum length of image which can be printed. Thus, typical offset presses provide printed matter having a fixed size. While smaller sized images may be transferred, this wastes space by leaving unprinted areas on the web, which unprinted areas must be discarded. Depending on the amount of space which is wasted, the economic costs may be substantial.
This waste factor has prevented offset printing press manufacturers from offering press designs that incorporate changeable plate cylinders having different diameters. Unless the blanket cylinder is also changed to match the plate diameter, the amount of wasted areas on the web will still be substantial. However, designing a press in which both the plate cylinder and blanket cylinder diameters are changeable significantly complicates the press design. Further, the labor and down time involved to change the press components adds further costs to the printer. Accordingly, these design limitations have to date restricted owners and users of offset press equipment from serving customers who have needs for various sizes of printed matter. Thus, there remains a need in this art for an offset press which can accommodate various sizes of printed matter without the waste associated with prior designs.